Field of the Invention
The present invention relates to a vehicle safety system equipped with a microcomputer and used to control the activation of air bags, seat belt tensioners and various other passenger protection devices.
Prior Art
Air bags, seat belt tensioners and various other types of vehicle safety devices have been developed for protecting vehicle passengers from impacts arising at the time of collision. The vehicle safety systems employing these passenger protection devices are designed to activate the air bags etc. when a control unit for processing signals from an acceleration sensor discriminates that a collision has occurred. A vehicle safety system of this type is therefore required to optimize the safety device activation control for the particular conditions of the collision. In recent years this has led to increased use of high-performance control units that utilize microcomputers for conducting the various types of data processing required for collision discrimination.
Two types of control programs are used in microcomputer-based control units of this type: an interrupt program which conducts high-priority processing such as collision detection on a periodic interrupt basis and a background program which conducts background processing of various low priority jobs such as power supply voltage monitoring, operation logging and failure diagnosis. Ordinarily these programs are appropriately executed in the microcomputer. Although the use of a microcomputer to process data in this manner enables response to various types of collision situations in the most appropriate manner, it also involves the risk of malfunctions occurring when the processing operations are not properly conducted owing to program runaway or the like.
Upon discrimination of a collision, it is necessary within a short period of from several milliseconds to several tens of milliseconds to supply the ignition element with a prescribed amount of ignition energy by passing a fixed amount of electric current through the ignition element for a period of several milliseconds. Detection of program runaway therefore has to be conducted within a very short time interval. The prior art systems are configured to monitor whether or not a program runaway has occurred by periodically outputting a monitor signal to the CPU (central processing unit) within the fixed-time interrupt routine.
This type of malfunction occurring during program execution in a microcomputer may, however, occur as a program runaway caused by a jump from background processing to an operation code thereof, by a jump from background processing to an operand thereof, or by various other causes. In such a system requiring highly dependable control operation, therefore, there is a need for a method for rapidly and reliably detecting program runaway and other problems arising in the microcomputer so as to be able to prevent malfunctioning of air bags and other passenger protection devices owing to program execution errors.